The inventors of this invention have carried out diligent research on the various methods for producing superconductive wire. For Nb3Al superconductive wire which has excellent magnetic field characteristics and which is excellent in warp resistance, the inventors have already been successful in producing a long superconductive wire having a superfine multi-core structure which is excellent in stability and alternating current characteristics, without losing the above-described characteristics of the wire (Japanese Patent No. 2021986).
However, generally when current flows through a superconductive wire, the current starts to flow from the surface of the wire and does not flow evenly. Therefore, there is the problem that the energy in the wire is released locally, and the superconductivity at that portion is destroyed to generating joule heat, and thus the destruction of the superconductive state spreads through the entire wire. This problem is particularly serious in the case of multi-core wires, and a complex of a stabilizing material for speeding up the cooling of the joule heat of the wire by bypassing the current at the portion where superconductivity has been destroyed, is necessary for practical use of the above-described Nb3Al superconductive wire. Also, in order to more effectively act this stabilizing material, it is desirable that the electrical resistance and thermal resistance at the border of the stabilizing material and the matrix material are extremely low.
In addition, a metal that has low electrical resistance and large thermal conductivity such as Cu, Ag and the like is generally used as the stabilizing material of the Nb3Al superconductive wire which is prepared by above-described the rapid heating and quenching and transformation method. The metal is complexed with a superconductive wire using (1) the Cu ion plating method (2) the internal Ag complex stabilization method or (3) the external Cu pressure contact complexing method.
In (1) Cu ion plating method, extremely thin Cu is disposed by the ion plating method on the Nb3Al superconductive wire which is obtained by carrying out the rapid heating and quenching process, and a strong connection is formed between the matrix material and the Cu, and the resultant wire is stabilized by further disposing Cu of a desired thickness using a plating method. In this method a favorable interface between the matrix material and stabilizing material is obtained. On the other hand however, there are problems in that many processes have to be carried out, and the method is very costly because processing is done in a vacuum chamber.
(2) The internal Ag complex stabilization method is one in which matrix material and Ag which has low reactivity are incorporated into the Nb3Al precursor as the stabilizing material. In this method, when Ag is subjected to the rapid heating and quenching process, it is necessary to support the Ag with matrix material so that the Ag wire does not break as a result of being exposed to a temperature in excess of the melting point. The Ag ratio cannot be too high and realistically the complex is prepared such that the Ag ratio is 0.4 or less. With this ratio, there is the problem that sufficient stability cannot be obtained for use as a superconductive wire. In addition, Ag is expensive and thus this method is costly. It is possible to use Cu whose melting point is high as the stabilizing material in this method, but because Cu reacts with the matrix material, there is the problem that it becomes necessary to deal with this.
n addition, (3) the external Cu pressure-contact complexing method is one in which the Nb3Al superconductive wire which is obtained by carrying out the rapid heating and quenching process is covered with copper tape, and then the round wire is machined to be flat and thus cause wire section deformation between the Cu and Nb (matrix material) and is thus a method in which there is pressure-contact of the matrix material and the Cu. (Japanese Patent Laid-Open No. 2000-113748). This method has the advantages of being very inexpensive and at the portions where there is much wire section deformation, an excellent connection can be obtained at the interface of the matrix material and the Cu. However, the ratio of Cu which is used as the stabilizing material is limited to the range of about 0.3˜0.6, and excellent connection is obtained for only about 60% of the entire interface and thus it is unsatisfactory in view of stability. The most notable disadvantage is that the superconductive wire is obtained as a flat wire, and it cannot be produced as a round wire for which demand is the highest.
As a result, the invention of this application was conceived in view of the above-described situation and solves the problems of the prior art by providing a method for producing Nb3Al superconductive wire as well as the Nb3Al superconductive wire obtained from said method in which: cost is comparatively low; both round and flat type wires can be produced; and superconductivity is more stable.